The ability to start a shell with all the right dependencies configured, without requiring root access or permanently installing new dependencies, seems like something useful.
https://nixos.wiki/wiki/Overlays#Overriding_a_version
With Nix flakes it's even easier and you can just mix and match inputs as you want. For example if you want to replace libfoo with your own fork you just do something like:
$ nix build --override-input libfoo /home/juser/src/libfoo
And the best part is that none of this requires any changes to your system. If you need multiple different specific versions at once, Nix can just do it, as every software is sitting around in /nix/store/{HASH} and there is no risk of anything colliding.